In a wireless communication system, particularly, in a mobile communication system, it has been studied to adapt a configuration in which functions of a base station are divided for two devices including a base band unit (BBU) and a remote radio head (RRH) and the BBU and the RRH are physically separated in order to increase flexibility of deployment of a base station. As one form of a function division scheme in the BBU and the RRH, a functional division scheme in which the BBU performs a function of a media access control (MAC) layer or a higher layer and an encoding function that is one function of a physical layer, and the RRH performs functions of the physical layer other than the encoding function as shown in FIG. 5 has been studied (Non-Patent Document 1). This function division scheme is called a split-PHY processing (SPP) scheme.
As a scheme of demodulating a radio signal received in a base station or a terminal, there is a soft decision demodulation scheme for outputting a signal bit obtained through demodulation as a ratio of a real number value called a likelihood indicating a probability of the signal bit being 0 or 1 instead of outputting the signal bit obtained through demodulation as a bit value of 0 or 1 (Non-Patent Document 2). In the soft decision demodulation scheme, the output is called a log likelihood ratio or LLR (Log Likelihood Ratio). In general, a greater positive value of the LLR indicates that a likelihood of the signal bit being 1 is higher, and a smaller negative value (with a greater absolute value) indicates that a likelihood of the signal bit being 0 is higher.
FIG. 6 is a diagram showing a configuration example of a wireless communication system to which an SPP scheme is applied. The wireless communication system includes a terminal 91, an RRH 92, and a BBU 93. The RRH 92 includes a radio frequency (RF) reception unit 921, a channel estimation unit 922, a demodulation unit 923, and an LLR quantization unit 924. The BBU 93 includes a decoding unit 931 and a high-level functional unit 932.
The RRH 92 and the BBU 93 perform predetermined settings before starting a process of receiving a radio signal that is transmitted from the terminal 91. Specifically, the high-level functional unit 932 transmits a control signal to the demodulation unit 923. The demodulation unit 923 sets a demodulation parameter for performing demodulation according to a state of a wireless transmission path on the basis of the control signal. Examples of the demodulation parameter include a parameter indicating a modulation scheme that is used for demodulation such as quadrature phase shift keying (QPSK) or 16 quadrature amplitude modulation (QAM), and a parameter indicating a coding rate that is used for demodulation such as ⅓ or ¾.
The radio signal transmitted from the terminal 91 is received by the RF reception unit 921 via the wireless transmission path. The RF reception unit 921 outputs a reference signal included in the received radio signal to the channel estimation unit 922, and outputs a data signal included in the received radio signal to the demodulation unit 923. The reference signal is used for extracting channel information of the wireless transmission path and includes a predetermined signal between the terminal 10 and the RRH 20. The data signal is to be sent to the BBU 93 and includes a sequence of signal bits. The channel estimation unit 922 estimates the channel information of the wireless transmission path on the basis of the reference signal, outputs the channel information to the demodulation unit 923, and feeds back the channel information to the BBU 93.
The demodulation unit 923 performs soft decision demodulation on the data signal using the demodulation parameter indicated by the control signal and the channel information output from the channel estimation unit 922. The demodulation unit 923 outputs an LLR value obtained by soft decision demodulation to the LLR quantization unit 924. Since the LLR value output from the demodulation unit 923 to the LLR quantization unit 924 is a real number value, the LLR quantization unit 924 transmits a value obtained through quantization performed on the LLR value to the BBU 93.
The decoding unit 931 receives the quantized LLR value from the RRH 92 and performs a decoding process on the received LLR value to obtain a signal bit. The decoding unit 931 outputs the obtained signal bit to the high-level functional unit 932 as information transmitted from the terminal 91.
The smaller the number of quantization bits for the LLR value obtained in the RRH 92, the smaller the amount of transmission between the RRH 92 and the BBU 93. When the number of quantization bits is merely made small, a quantization error of the LLR value becomes large. Therefore, as an LLR quantization scheme, there is a scheme that collects samples of the LLR value, obtains a statistical distribution of the LLR value, and determines an optimal quantization threshold value and an optimal quantization level for the statistical distribution at a predetermined number of quantization bit (Non-Patent Document 3).